The New South Wales Ambulance Service has conceded it left a triple-0 caller waiting for 20 minutes earlier this year, but says it was experiencing an "avalanche of calls" at the time.

The Ambulance Service says it regrets leaving the emergency call unanswered for 20 minutes in August, but the director of its controls division, Jamie Vernon, says the incident is highly unusual.

"Our average answer times for all the the triple-0 calls is somewhere between six and eight seconds," he said.

"On that particular day we did have an avalanche of calls.

"During that particular period of time during August we were experiencing a greater degree of busyness than any other time, in fact it was up quite considerably each day during August and it's come back down."

"Everyone knows that August and September is the busiest time for the health system.

"It's simply a matter of employing enough staff to meet peak demand.

"Every time the staff get overwhelmed, the health system claims the demand is unprecedented."

He has pointed to an auditor-general's report showing demand for ambulances has grown at 4 per cent per year, but in the past five years the number of call centre staff has only gone up a total of 7 per cent.